Wells are generally drilled into the ground to recover natural deposits of hydrocarbons and other desirable materials trapped in geological formations in the Earth's crust. A well is typically drilled by advancing a drill bit into the earth. The drill bit is attached to the lower end of a “drill string” suspended from a drilling rig. The drill string is a long string of sections of drill pipe that are connected together end-to-end to form a long shaft for driving the drill bit further into the earth. A bottom hole assembly (BHA) containing various instrumentation and/or mechanisms is typically provided above the drill bit. Drilling fluid, or mud, is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and it carries drill cuttings back to the surface in the annulus between the drill string and the borehole wall.
In conventional drilling, a well is drilled to a selected depth, and then the wellbore is typically lined with a larger-diameter pipe, usually called casing. Casing typically consists of casing sections connected end-to-end, similar to the way drill pipe is connected. To accomplish this, the drill string and the drill bit are removed from the borehole in a process called “tripping.” Once the drill string and bit are removed, the casing is lowered into the well and cemented in place. The casing protects the well from collapse and isolates the subterranean formations from each other. After the casing is in place, drilling may continue.
Conventional drilling typically includes a series of drilling, tripping, casing and cementing, and then drilling again to deepen the borehole. This process is very time consuming and costly. Additionally, other problems are often encountered when tripping the drill string. For example, the drill string may get caught up in the borehole while it is being removed. These problems require additional time and expense to correct.
The term “casing drilling” refers to the use of a casing string in place of a drill string. Like drill string, a chain of casing sections are connected end-to-end to form a casing string. The BHA and the drill bit are connected to the lower end of a casing string, and the well is drilled using the casing string to transmit drilling fluid, as well as axial and rotational forces, to the drill bit. Upon completion of drilling, the casing string may then be cemented in place to form the casing for the wellbore. Casing drilling enables the well to be simultaneously drilled and cased.
FIG. 1 shows a prior art casing drilling operation. A drilling rig 100 at the surface is used to rotate a casing string 110, or drill string comprised of casing. The casing string 110 extends down into borehole 102. A BHA 111 is connected at the lower end of the casing string 110. A drill bit 114 and an underreamer 112 are also provided at the lower end of the BHA 111.
When using casing drilling, the drill bit 114, underreamer 112, and the BHA 111 are typically sized so that they may be retrieved up through string 110 when drilling has been completed or when replacement and maintenance of the drill bit 114 is required. The drill bit 114 drills a pilot hole 104 that is enlarged by an underreamer 112 so that the casing string 110 will fit into the drilled hole 102. A typical underreamer 112 can be positioned in an extended and a retracted position. In the extended position, the underreamer 112 is able to enlarge the pilot hole 104 to a size larger than the casing string 110, so that the casing string will be able to fit into the drilled wellbore. In the retracted position (not shown), the underreamer 112 is retracted so that is able to travel through the inside of the casing string 110.
Casing drilling eliminates the need to trip the drill string before the well is cased. The BHA may simply be retrieved by pulling it up through the casing string. The casing string may then be cemented in place, and then drilling may continue. This reduces the time required to retrieve the BHA and eliminates the need to subsequently run casing into the well.
Another aspect of drilling is called “directional drilling.” Directional drilling is the intentional deviation of the wellbore from the path it would naturally take. In other words, directional drilling is the steering of the drill string so that it travels in a desired direction.
Directional drilling is advantageous in offshore drilling because it enables many wells to be drilled from a single platform. Directional drilling also enables horizontal drilling through a reservoir. Horizontal drilling enables a longer length of the wellbore to traverse the reservoir, which increases the production rate from the well.
One method of directional drilling uses a BHA that includes a bent housing and a mud motor. A bent housing apparatus is described in U.S. Pat. No. 5,117,927, which is assigned to the assignee of the present invention. That patent is incorporated by reference in its entirety. An example of a bent housing 200 is shown in FIG. 2A. The bent housing 200 includes an upper section 203 and a lower section 204 that are formed on the same drill pipe, but are separated by a bend 201. The bend 201 is a permanent bend in the pipe.
With a bent housing 200, the drill string is often not rotated from the surface. Instead, the drill bit 205 is pointed in the desired drilling direction, and the drill bit 205 is rotated by a mud motor (not shown) in the BHA. A mud motor converts some of the energy of the mud flowing down through the drill pipe into a rotational motion that drives the drill bit 205. Thus, by maintaining the bent housing 200 at the same azimuthal position with respect to the borehole, the drill bit 205 will drill in the desired direction.
When straight drilling is desired, the drill string, including the bent housing 200, is rotated from the surface. The drill bit 205 angulates with the bent housing 200 and drills a slightly overbore, but straight, borehole (not shown).
Another method of directional drilling includes the use of a rotary steerable system (“RSS”). In an RSS, the drill string is rotated from the surface, and downhole devices cause the drill bit to drill in the desired direction. Rotating the drill string greatly reduces the occurrences of the drill string getting hung up or stuck during drilling.
Generally, there are two types of RSS's point the bit systems and push the bit systems. In a point the bit system, the drill bit is pointed in the desired direction of the borehole deviation, similar to a bent housing. Embodiments of a point the bit type system are described in U.S. patent application Ser. No. 10/122,108, published on Nov. 28, 2002, as Publication No. 2002/0175003. That application is assigned to the assignee of the present invention, and it is incorporated by reference in its entirety. A point the bit system works in a similar manner to a bent housing because a point the bit system typically includes a mechanism for providing a drill bit alignment that is different from the drill string axis. The primary differences are that a bent housing has a permanent bend at a fixed angle, and a point the bit RSS has an adjustable bend angle that is controlled independent of the rotation from the surface.
FIG. 2B shows a point the bit system 210. A point the bit RSS 210 typically has an drill collar 213 and a drill bit shaft 214. The drill collar includes an internal orientating and control mechanism that counter-rotates relative to the drill string. This internal mechanism controls the angular orientation of the drill bit shaft 215 relative to the borehole.
The angle θ between the drill bit shaft 215 and the drill collar 213 may be selectively controlled. The angle θ shown in FIG. 2B is exaggerated for purposes of illustration. A typical angle is less than 2 degrees.
The “counter rotating” mechanism rotates in the opposite direction of the drill string rotation. Typically, the counter rotation is at the same speed of the drill string rotation so that the counter rotating section maintains the same angular position relative to the inside of the borehole. Because the counter rotating section does not rotate with respect to the borehole, it is often called “geo-stationary” by those skilled in the art. In this disclosure, no distinction is made between the terms “counter rotating” and “geo-stationary.”
In a push the bit system, devices on the BHA push the drill bit laterally in the direction of the desired borehole deviation by pressing on the borehole wall. Embodiments of a push the bit type system are described in U.S. patent application Ser. No. 10/140,192, published on Dec. 5, 2002, as Publication No. 2002/0179336. That application is assigned to the assignee of the present invention, and it is incorporated by reference in its entirety.
A push the bit system typically uses either a rotating or non-rotating stabilizer and pad assembly stabilizer. When the borehole is to be deviated, a actuator presses a pad against the borehole wall in the opposite direction from the desired deviation. The result is that the drill bit is pushed in the desired direction.
FIG. 2C shows a typical push the bit system 220. The drill string 223 includes a collar 221 that includes a plurality of extendable and retractable pads 226. Because the pads 226 are disposed in the non-rotating collar 221, they do not rotate with respect to the borehole (not shown). When a pad 226 is extended into contact with the borehole (not shown) during drilling, the drill bit 225 is pushed in the opposite direction, enabling the drilling of a deviated borehole.
What is needed is a technique which captures the benefits of various RSS's for use in casing drilling applications. It is desirable that such a technique would permit drilling and casing with the same tool, while permitting directional drilling. It is further desirable that such a system employ downhole drilling tools capable of drilling to optimize the casing operation as well as the drilling operation. The present invention is provided to meet these and other needs.